The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications, is well known. Such acoustic measurement systems are utilized in a variety of downhole applications including, for example, borehole caliper measurements, measurement of drilling fluid properties, and the determination of various physical properties of a formation. In one application, acoustic waveforms may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced apart receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to as slownesses) of a formation.
It will be appreciated that the terms slowness and velocity are often used interchangeably in the art. They will likewise be used interchangeably herein with the understanding that they are inversely related to one another and that the measurement of either may be converted to the other by simple and known mathematical calculations. Additionally, as used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) whereas LWD typically refers to measurements taken for the purpose of analysis of the formation and surrounding borehole conditions. Nevertheless, these terms are herein used synonymously and interchangeably.
In the analysis of acoustic logging measurements, the received acoustic waveforms are typically coherence processed to obtain semblance data which may be displayed on a time-slowness plot. In a time-slowness plot, also referred to as a slowness-time-coherence (STC) plot or a semblance plot, a set of several signals from the array of acoustic receivers is processed with the incorporation of separate time shifts for each received signal. The separate time shifts are based on a slowness value assumed for the purpose of processing the waveforms. The processing provides a result, known as coherence, which can signify the presence of a discernable signal received by the separate receivers. In this manner compressional and shear wave arrivals can be discerned in the received waveforms. The compressional and shear wave slownesses so determined are related to compressive and shear strengths of the surrounding formation, and thus provide useful information about the formation.
In acoustically slow formations, in which the velocity of shear waves is less than the speed of sound in the drilling fluid (mud), shear wave slowness determination is known to be complicated by poor transmission of shear wave energy across the boundary between the formation and the borehole. Various techniques have been developed for determining shear wave slowness in acoustically slow formations. These techniques commonly involve exciting borehole guided waves (e.g., monopole, dipole, or quadrupole modes). The shear wave slowness is then estimated from the guided wave velocity of these modes.
To be useful, the techniques used must excite guided waves of sufficient strength for propagation and detection. In some boreholes this can be problematic, for example, if the borehole diameter is much larger than the tool body, and the tool is located near the center of the borehole. Excitation of sufficiently strong guided waves can also be a problem if the borehole is deviated (and/or the tool is decentralized in a large borehole) and the transmitter and receiver face away from the closest borehole wall.
One way to address this difficulty is to increase the strength of the acoustic transmitter, increase the sensitivity of the receivers, or both. However, because acoustic energy may also be transmitted from the transmitter to the receivers through the tool body as tool noise, these modifications to the transmitter power and/or receiver sensitivity also increase tool noise. Another option may be to decentralize the tool in the borehole and include advanced processing capability in the tool, for example, that uses tool face angle information to determine when the transmitter standoff is small. In this way, transmitter firing times could be selected for when standoff is optimum. But this tends to be a complex solution and adds to the expense of the tool.
Another problem commonly encountered in shear wave slowness determination is the aliasing of the compressional wave signal in acoustically slow formations. This “aliasing effect” may mask or mimic the presence of a shear wave signal, and tends to be particularly harmful when the alias is close to an expected shear wave arrival time. The effect of aliasing can be diminished by reducing the distance between acoustic receivers, but this also reduces array coverage which can increase uncertainty in coherence analysis, unless more receivers are added, at more expense.
Yet another difficulty encountered in shear wave slowness determination is that guided wave propagation tends to be highly dispersive in LWD applications. Although STC analysis is widely used, dispersive effects in the received waveforms can reduce the reliability of the STC analysis. By dispersive it is meant that the measured guided wave slowness is frequency dependent. The amount of dispersion depends on many factors in addition to the shear slowness of the formation. These factors include, for example, tool body properties, eccentricity, borehole diameter, propagation frequency, compressional slowness, and mud density and slowness. In order to obtain a suitably accurate shear slowness value, processing that relies upon values for these factors is required. This processing is commonly referred to in the art as “dispersion correction”. In many applications, values for each of these other factors are not accurately known, which can in turn lead to errors in the shear slowness estimate. Moreover, in some formations large dispersion correction may be required, which can further contribute to uncertainty in the shear wave slowness.
Another drawback with the aforementioned techniques is that logging while drilling tools configured for transmitting and/or receiving the relatively pure acoustic modes mentioned earlier require highly complex transmitter and/or receiver configurations, which tend to be expensive. For example, transmitters configured to produce a pure acoustic mode typically include numerous (e.g., four, eight, or even more) distinct transducer elements deployed about the circumference of the tool body. In order to produce a pure mode and to suppress other modes, highly precise phasing (timing) of the various transducers is typically required. The difficulty in generating such acoustic signals is also known to be further exacerbated by tool eccentricity in the borehole (e.g., in highly deviated wells in which the tool typically lies on the low side of the borehole). Moreover, the use of such complex transmitters and receivers in severe downhole conditions including extreme temperatures and pressures, severe mechanical shocks and vibrations (up to 650 G per millisecond) tends to reduce tool reliability.
Therefore, there exists a need for an improved downhole acoustic logging tool suitable for use in determining a shear wave velocity of a subterranean formation. In particular, it will be appreciated that a logging while drilling tool that can accommodate downhole geometries that tend to inhibit borehole guided wave generation, propagation, and/or detection would be highly advantageous, since many of the above stated disadvantages would thus be obviated.